The expansion costs $20 USD and is not covered by the game’s Season Pass.

As an aside, Leaders Jerome and Arbiter (Ripa ‘Moramee, HW1 Arbiter) are playable at E3 right now in the Terminus Firefight mode. Leaders Jerome and Arbiter will be coming to Halo Wars 2 in late June for Seasons Pass owners or $5.99 USD/each.